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Assessment of the Pore Pressure Distribution in The Presalt Santos Basin, Brazil: A Regional Approach for Prospect De-Risking

Abstract

The presalt area of the Santos Basin, located in the Central Atlantic Margin offshore Brazil, is one of the most prolific oil province discovered recently. During the last decade structures of several billion barrels of oil have been discovered in water depths ranging from 1500 to more than 2000 m (Lula, Iara, Sapinhoa). The petroleum system is comprised by microbial carbonates of the Sag Member and the Coquinas Unit sealed by Aptian evaporites ensuring a perfect seal (reaching more than 2000 m thickness in the Central portion of the basin). Toward the platform salt welding occurred allowing pre- to post-salt hydrocarbon migration. Due to evaporite occurrence, the pressure regime is a critical operational aspect. Abnormal pressures were expected in the pioneer wells. A contrario, they registered a normal pore pressure trend whereas further wells closer to the weld area presented strong overpressure. The observed pressure distribution in the basin did not correspond to the expectation and the pressure regime in the Santos presalt province remains to be properly understood. The statistical approach still prevails other than understanding the geological mechanisms able to explain the observed pressure data. This study is an integrated and regional approach aiming a first proposal for pore pressure distribution and related mechanism description in the presalt sequence of the Santos basin. A regional 3D-earth model covering most of the presalt Santos basin and representative of the different salt tectonic, structural and stratigraphic styles has been built for basin modeling purpose. Data from about 40 exploration and production wells have been used to properly calibrate the pressure and characterize the petrophysical presalt facies. Regional porosity-depth curves were also derived from these data and implemented into the 3D model. Relationships between petrophysical properties, volcanism occurrence and pressure are also discussed, illustrating the strengths and weaknesses of the basin modeling approach in this study. Pore pressure regime in the Santos Basin presalt sequence is mainly driven by the Sag facies variation (thus the paleogeography) and the occurrence and timing of volcanism steps. Based on extensive data compilation, regional paleogeographic maps and 3D basin modeling simulations this study proposes a geological understanding and predictive model of the pressure distribution for future exploration and appraisal presalt targets.